Calendar methods of fertility regulation: a rule of thumb

Suppose that in each menstrual cycle a group of couples abstains for k days from intercourse, otherwise allowed in any other phase of the cycle. In a given cycle, the conception probability will depend on k, on the location within the cycle of the period of abstinence and on the pattern of intercourse episodes. An elaboration of this as-sumption on groups of cycles for which the relevant documentation is available gives idea of its consequences. For each group, under various conditions, an average concep-tion probability in a cycle is estimated. For that purpose, several hypotheses are made: about the size of k; about the location of the period of abstinence within the cycle; about frequency of intercourse outside that phase and the pattern of fecundability on different days of the menstrual cycle. Two fecundability models have been chosen from the literature and the calculations have been made on groups of cycles conveniently selected from a large data base of empirical observations. The results of the exercise give information of some relevance for fertility regulation through the use of calendar methods